Ogionheim is hidden away somewhere in the Brokenstone Hills on the south shore of Lake Azalar, three days march north-northeast of Brisea. (If you don't have want to explore: http://www.dungeonraider.net/156248)

Notably the final rewards from the main quest line that could be started from the city daily quest interface. The highest level event dungeon may reward you with a prestige dungeon decoration too, the last events I did was for Halloween and Christmas. I'm thinking of doing an event for the chinese new year. Do check the main dungeon listing, event dungeons may appear at the bottom of the screen.